Studies of electrodes modified with zeolites and poly(4 nitro 2 phenylenediamine) and their composite 3

CONTENTS Pages Acknowledgements i Table of Contents ii Summary vii List of Figures ix List of Tables xix Chapter I Introduction 1 1.1 Preparation of chemically modified electrodes

ACKNOWLEDGEMENTS

I am indebted to my supervisor, Dr Khoo Soo Beng for his utmost patience, constant guidance and encouragement throughout the course of this project

Special thanks are given to my fellow researchers in the laboratory,

Ms Fathima Shahitha and Chen Fang for their selfless assistance and valuable suggestions given to me during this project

The provision of Research Scholarship from the National University of Singapore is gratefully acknowledged

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CONTENTS

Pages

Acknowledgements i

Table of Contents ii

Summary vii

List of Figures ix

List of Tables xix

Chapter I Introduction 1

1.1 Preparation of chemically modified electrodes 4

1.1.1 Electrodes modified with monomolecular layers 4

1.1.1.1 Chemisorption 4

1.1.1.2 Covalent bonding 4

1.1.1.3 Hydrophobic Layers 5

1.1.2 Electrodes modified with multimolecular layers 6

1.1.2.1 Polymers 6

1.1.2.2 Inorganic films 10

1.1.3 Electrodes modified with spatially defined and heterogeneous layers 11

1.2 Characterization and analysis of chemically modified electrodes 15

1.2.1 Electrochemical methods 15

1.2.2 Spectroscopy and microscopy methods 18

1.2.3 Quartz crystal microbalance 19

1.3 Applications of chemically modified electrodes 21

1.3.1 Chemical sensors 21

1.3.2 Energy-producing devices 22

1.3.3 Electrochromic devices 22

1.3.4 Fundamental chemistry 23

1.4 The objectives of the thesis 25

References 28

Chapter II Studies of zeolite modified electrodes fabricated by electrophoretic deposition 35

2.1 Introduction 36

2.2 Experimental 39

2.2.1 Reagents 39

2.2.2 Apparatus 40

2.2.3 Procedure 40

2.3 Results and discussion 43

2.3.1 Zeolite modified electrode fabricated by dc voltage EPD 43 2.3.1.1 Effect of the supporting electrolyte concentration 43

2.3.1.2 Effects of dc voltage and time 44

2.3.1.3 Effects of different supporting electrolytes and solvent 48

2.3.2 Zeolite modified electrode fabricated by pulsed voltage EPD 52

2.3.3 Stability and applicability of zeolite modified electrode

fabricated by pulsed EPD 56

2.4 Conclusions 62

References 63

Chapter III Electropolymerization of 4-nitro-1,2-phenylenediamine and

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electrochemical studies of poly(4-nitro-1,2-phenylenediamine) films

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3.1 Introduction 66

3.2 Experimental 68

3.2.1 Reagents 68

3.2.2 Apparatus 68

3.2.3 Procedure 68

3.3 Results and discussion 70

3.3.1 Electropolymerization of 4NoPD 70

3.3.2 Electrochemical characterization of P4NoPD films 74

3.3.2.1 Films formed using different cycles and different

supporting electrolytes 74

3.3.2.2 Scan rate studies 76

3.3.2.3 Stability of P4NoPD film 76

3.3.2.4 pH effect 77

3.3.3 Reduction of the nitro-groups of 4NoPD monomer 80

3.3.4 Electrochemical behaviors of the nitro-groups of P4NoPD

film 83

3.4 Conclusions 93

References 94

Chapter IV Electrochemical impedance spectroscopic studies of poly(4-nitro

1,2-phenylenediamine) film modified electrodes 97

4.1 Introduction 98

4.2 Experimental 100

4.2.1 Reagents 100

4.2.2 Apparatus 100

4.2.3 Procedure 100

4.3 Results and discussion 102

4.3.1 P4NoPD film in 0.50 M H2SO4 102

4.3.2 P4NoPD films formed in different numbers of cycles 109

4.3.3 pH effect 113

4.3.4 Effect of the reduction of nitro-groups of P4NoPD film 114

4.4 Conclusion 122

Reference 123

Chapter V Studies of electrodes modified with poly(4-nitro-1,2-

phenylenediamine) / zeolite composite 125

5.1 Introduction 126

5.2 Experimental 128

5.2.1 Reagents 128

5.2.2 Apparatus 128

5.2.3 Procedure 129

5.3 Results and discussion 131

5.3.1 Characterization of the composites in the media (pH=2) 133

5.3.2 Characterization of the composites in the media containing

redox active species 137

5.3.2.1 Effect of the amount of zeolites 142

5.3.2.2 pH effect 145

5.3.2.3 Accumulation of Fe(CN)63- 147

5.3.2.4 Effect of redox active species 147

5.4 Conclusions 154

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References 155

List of publications 158

SUMMARY

In recent years, a number of works have devoted to the preparation, characterization, and electrochemical behavior of chemically modified electrodes (CMEs) In the 1960s, interest arose in the modification of electrode surfaces by covalent attachment of monolayers to electrode surfaces Electrodes modified with thicker polymeric films and inorganic layers were introduced later Here we discuss the chemical and physical routes for the preparation of CMEs and the electrochemical and other consequences of this

Early fabrication of zeolite modified electrodes (ZMEs) generally has been plagued by poor reproducibility, lack of mechanical robustness in a stirred solution, and nonideal electrochemical behavior Therefore, controllable formation of zeolite thin film needs new processing schemes to improve quality and reproducibility We have shown here the applicability of a novel approach to controllable zeolite deposition on electrode surface by pulsed electrophoretic deposition While dc electrophoretic deposition also affords control of zeolite deposition, it is less tuneable and convenient as different solutions have to be used and, more importantly, at the expense of electrode surface integrity In contrast, pulsed EPD is simple, yet more powerful and convenient to give control of the amount of zeolite deposited (from sub-monolayer to multilayer) by tuning the pulse widths, heights and number of pulses With regard to stability of ZMEs, it is our view that long term stability, if the ZME is going to be used and reused with washing, storage and exposure to the atmosphere, is not viable without some means of support/anchoring

Conducting polymers, in particular electrodes modified with conducting polymer film, have enjoyed initial success and recently stimulated extensive activities New types of polymer materials are still being developed to meet the challenges in the

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wide range of areas Here, we have shown the anodic electropolymerization of

4-nitro-1,2-phenylenediamine (4NoPD), which is dependent on the acidity as well as the

nature of anions in supporting electrolyte It is essential to know and understand the

properties of poly(4-nitro-1,2-phenylenediamine) (P4NoPD) film (i.e film

conductivity, charge-carrier transfer parameters, capacitance, etc) and the effects of various factors (pH, film thickness, nitro-groups) on these properties Therefore, we

applied the electrochemical studies of P4NoPD film under different conditions The P4NoPD film was confirmed to have conductivity which was highly electroactive

behavior, dependent on film thickness, acidity of the solution and the redox state of the film Also, film properties (film resistance, low-frequency capacitance, and charge-carrier diffusion coefficient) were influenced by the presence of the electron-withdrawing nitro-groups

Finally, we have shown the applicability of a novel approach to composite

P4NoPD and EPD zeolite film on electrode surface The results proved the

dependence of the properties of the composite film on the amount of zeolites In addition, the electrochemistry of the composite films in the presence of different redox active probes was studied under the present experimental conditions All of these techniques give the consistent result that the novel properties of composite films

can be derived from the successful combination of the characteristics of P4NoPD and

zeolite 13X

List of Figures

Pages Figure 2.1 SEM images of dc voltage EPD fabricated in 3.00 g l-1 suspension of 46

zeolite 13X, 10-5 M KNO3 for 30 min:

(a) E=1.9 V; (b) E=2.0 V; (c) E=2.2 V; (d) E=2.5 V

Figure 2.2 CVs (50 mV s-1) in 1.00 mM Fe(CN)63-, 1.00 M KCl: (a) bare 47

GCE, no EPD; (b) EPD in 3.00 g l-1 suspension of zeolite

13X, 10-5 M KNO3 at 2.2 V for 30 min; (c) EPD in 3.00 g l-1

suspension of zeolite 13X, 10-5 M KNO3 by pulsed voltage

(E1=0 V, t1=50 s, E2=2 V, t2=200 s, 10 steps)

For (b) and (c), the CVs were obtained after removing the

deposited zeolite particles by rinsing strongly and copiously

with Millipore water While (b) and (c) are for close to

monolayer depositions, similar results were obtained for all

levels of depositions studied

Figure 2.3 (a) Typical SEM image of dc EPD fabricated in 3.00 g l-1 49

suspension of zeolite 13X, 10-5 M KNO3 at +4.50 V for 40 s;

(b) amplified image of the selected area

Figure 2.4 Pulse Waveform 53

Figure 2.5 SEM images of pulsed EPD fabricated in 3.00 g l-1 suspension 55

of zeolite 13X, 10-5 M KNO3 for 30 min.: (a) E1=0 V, t1=50 s,

E2=2 V, t2=190 s; (b) E1=0 V, t1=50 s, E2=2 V, t2=200 s; (c)

E1=0 V, t1=20 s, E2=2 V, t2=200 s; (d) E1=0 V, t1=30 s, E2=2

V, t2=190 s; In all cases, the number of steps was 10

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Figure 2.6 CVs (50 mV s-1) at ZME (E1=0 V, t1=20 s, E2=2 V, t2=200 s, 58

10 steps, multilayer) in 1.00 mM Fe(CN)63-, 0.10 M KNO3

after accumulation in the same solution (stirred): first scan

(⎯); second scan ( -) The CVs of (a)-(g) are time series, and

(h) CV at the bare GCE

Figure 3.1 CVs (50 mV s-1) at the Au electrode in 0.50 M H2SO4 solution, 71

containing 4.50 mM 4NoPD, between -0.15 V and +1.10 V

The cycles shown are numbers 1, 10, 15, 20, and 25 (being the

last cycle)

Figure 3.2 Plot of i vs t-1/2 for the chronoamperometric response at the 73

Au electrode in 0.50 M H2SO4 containing 0.05 mM 4NoPD

The potential was stepped from 0.4 V to 1.0 V

Figure 3.3 (a) CVs (50 mV s-1) of three P4NPoD films on Au electrodes 75

(electropolymerizations were performed in 0.50 M H2SO4

containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution:

25-cycles film (―); 40-25-cycles film ( ); 70-25-cycles film (····)

(b) CVs (50 mV s-1) in 0.50 M H2SO4 for three P4NPoD films,

electropolymerized in different H2SO4-Na2SO4 solutions

containing 4.50 mM 4NoPD on Au electrodes: 0.10 M H2SO4

-0.40 M Na2SO4 (―); 0.20 M H2SO4-0.30 M Na2SO4 ( );

0.50 M H2SO4 (····)

Figure 3.4 Plots of anodic peak currents ip.a of P4NoPD films on Au 78

electrodes (25-cycles, fabricated in 4.50 mM 4NoPD in 0.50 M

H2SO4) under different time and storage regimes:

(a) continuous cycling in 0.50 M H2SO4; (b) in between CVs in

0.50 M H2SO4, electrode was stored in 0.50 M H2SO4; (c) in

Millipore water

In all cases, the scan rate was 50 mV s-1

Figure 3.5 Plots of anodic peak potential Ep.a (mV) of P4NoPD film at 79

different pH values, scan rate: 50 mV s-1 in sulfate solution (¡)

and in phosphate buffer (c) The ion strength of these

solutions is ca 0.2

Figure 3.6 CVs of the reduction of the nitro-groups of 4.50 mM 4NoPD monomer 84

in 0.50 M H2SO4at the GCE for 3 successive cycles: first cycle

(―); second cycle ( ); third cycle (····) The scan rate was 50

mV s-1

Figure 3.7 CVs of the reduction of nitro-groups of P4NoPD film on GCE 85

(formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4)

in 0.50 M H2SO4 for 3 successive cycles: first cycle (―);

second cycle

( ); third cycle (····) The scan rate was 50 mV s-1

Figure 3.8 Redox peaks for the P4NoPD film (25 cycles in 4.50 mM 87

4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4; comparison of

fresh film (―) with a film in which nitro-groups had been

reduced in 0.150 M H2SO4 (see Figure 3.7) ( )

CVs were obtained at 50 mV s-1

Figure 3.9 (a) CVs (50 mV s-1) of the P4NoPD film (25 cycles in 4.50 mM 88

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4NoPD in 0.50 M H2SO4) on GCE in: 0.10 M NaOH (―); 1.00

M NaOH ( )

(b) CVs (50 mV s-1) of the P4NoPD film (as above) on GCE in

0.50 M H2SO4, comparison of fresh film (―) and film after

reduction of nitro-groups in 1.00 M NaOH ( )

Figure 3.10 (a) CV (50 mV s-1) of P4NoPD film (25 cycles in 4.50 mM 91

4NoPD in 0.50 M H2SO4) on GCE in 0.10 M Na2SO4

(pH = 5.41): first cycle (―); second cycle ( ); (3) third cycle

(····)

(b) CVs (50 mV s-1) of P4NoPD films (as above) on GCE in

the sulfate media of different pH for first cycle: pH = 0.30 (―);

pH= 5.41 (-·-·-·-); pH = 7.90 ( ); pH = 11.00 (····)

Figure 4.1 The impedance spectra of P4NoPD film (formed with 25 103

cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) on GCE in 0.50

M H2SO4: (a)-0.20 V; (b) 0.00 V; (c) 0.20 V

A 5 mV amplitude sine wave at the frequency of 20K was used

as perturbation signal

Figure 4.2 The equivalent circuit for the polymer modified electrode: 104

(a) simple model; (b) modified model [23]

Ru is the ohmic resistance of the electrolyte and the surface

layer, Cdl is the double-layer capacitance, Rct is the charge

transfer resistance, ZD is the diffusion impedance, Co is a

constant capacitance and CPE is a constant phase element

Figure 4.3 Comparison of the measured impedance spectra (points) 107

obtained from Figure 4.1(a) with simulated curves based on the

model in Figure 4.2(b) (solid line): (a) Nyquist plot; (b) Bode

plot

The values used for the simulations: Ru (5.2 ohm); Cdl (2.1 µF);

Rct (18 ohm); Co (7.8 mF); CPE, Zcpe = (1/ 6.2 × 10-4) (jω) –0.31;

ZD = (510/(jω)0.5) coth(2 × 10-2 (jω)0.5)

Figure 4.4 (a) Plot of film resistance (Rp) for three P4NPoD films on GCEs 111-112

(formed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in

0.50 M H2SO4 solution

(b) Plot of the charge transport diffusion coefficient (D) for

containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution

(c) Plot of low-frequency capacitance (CL) for three P4NPoD

films on GCEs (formed in 0.50 M H2SO4 containing 4.50 mM

4NoPD) in 0.50 M H2SO4 solution

For all these diagrams, a 5 mV amplitude sine wave at the

frequency of 20K was used for resistance measurements:

25-cycles film (♦); 40-25-cycles film (•); 70-25-cycles film (c)

Figure 4.5 (a) Plot of the low-frequency capacitance (CL) for P4NoPD film 115-116

on GCE (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M

H2SO4) in different H2SO4-Na2SO4 solutions

(b) Plot of film resistance (Rp) for P4NoPD film on GCE

(formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4)

in different H2SO4-Na2SO4 solutions on GCE For resistance

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